Asynchronous programming with Python using Promises

While I was working on a Python project, I needed asynchronous programming and noticed that I couldn't find a lightweight library to do so. So I made myself a promise...

from threading import Thread
from threading import Event
 
class Deferred(object):
    def __init__(self):
        self._event = Event()
        self._rejected = False
        self._result = None
 
    def resolve(self, value):
        self._rejected = False
        self._result = value
        self._event.set()
 
    def reject(self, reason):
        self._rejected = True
        self._result = reason
        self._event.set()
 
    def promise(self):
        promise = Promise(self)
        return promise
 
 
class Promise(object):
    def __init__(self, deferred):
        self._deferred = deferred
 
    def then(self, resolved = None, rejected=None):
        defer = Deferred()
 
        def task():
            try:
                self._deferred._event.wait()
                if self._deferred._rejected:
                    result = self._deferred._result
                    if rejected:
                        result = rejected(self._deferred._result)
                        
                    defer.reject(result)
                else:
                    result = self._deferred._result
 
                    if resolved:
                        result = resolved(self._deferred._result)
 
                    defer.resolve(result)
            except Exception as ex:
                defer.reject(ex.message)
 
        Thread(target=task).start()
 
        return defer.promise()
 
    def wait(self):
        self._deferred._event.wait()
 
    @staticmethod
    def wait_all(*args):
        for promise in args:
            promise.wait()

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And with that in place I can now write code like this:

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controller = Controller("COM7")
controller.start()
 
message1 = DiscoveryRequest()
promise1 = controller.send(message1).then(on_ready, on_error)
 
message2 = RequestNodeInfoRequest(1)
promise2 = controller.send(message1).then(on_ready, on_error)
 
Promise.wait_all(promise1, promise2)
 
controller.stop()

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